The present invention relates to a negative amplifier circuit sitable for use in a high frequency amplifier, a high gain amplifier or the like which is used, for example, in a radio receiver, a television receiver, a satellite broadcasting receiver, a video recorder, a mobile communication apparatus or the like.
In a high frequency circuit and a high gain amplifier circuit which are used, for example, in a television receiver, a gain control amplifier circuit for example as shown in FIG. 1 has been used. In other words, FIG. 1 shows the arrangement of a gain control amplifier circuit.
In FIG. 1, there is provided a differential pair consisting of transistors 61a, 61b and input signal sources 62a, 62b of voltages.+-.V.sub.IN are connected to the bases of these transistors 61a, 61b respectively. The emitters of the transistors 61a, 61b are grounded by way of current sources 63a, 63b and an emitter resistor 64 of a resistance value R.sub.E is connected between these emitters.
A current control circuit consisting of transistors 66a, 66b, 67a, 67b which are controlled by a control voltage source 65 of voltages.+-.V.sub.C1 is connected to the collectors of the transistors 61a, 61b. The collectors of the transistors 66a, 66b in this current control circuit are connected to a voltage source V.sub.CC by way of collector resistors 68a, 68b of a resistance value R.sub.C and output signal terminals 69a, 69b of voltages.+-.V.sub.0 are lead out from the collectors of the transistors 66a, 66b.
By the way, in the gain control amplifier as shown in FIG. 1, the maximum gain thereof is determined by the ratio of the resistance value R.sub.C of the collector resistors 68a, 68b to the resistance R.sub.E of the emitter resistor 64. Moreover, the maximum S/N is determined by the resistance value R.sub.C of the collector resistors 68a, 68b.
First, an explanation of the maximum gain will be provided. If the resistance value R.sub.C of the collector resistors 68a, 68b is made larger to increase the gain in the above mentioned circuit, the circuit will act as a low pass filter (LPF) due to the floating capacities existing in the collectors of the transistors 66a, 66b. Accordingly, the gain vs. frequency characteristic of the circuit will be attenuated at higher frequencies.
However, if the resistance value R.sub.C of the collector resistors 68a, 68b is selected to be made smaller in order to extend the frequency characteristic, the gain is lowered. Accordingly, in order to realize an increased gain without an LPF effect, it is necessary to decrease the resistance value R.sub.E of the emitter resistor 64. However, if R.sub.E is decreased, it is necessary to increase the bias current to maintain the dynamic range. In that case, the load resistance should be re-estimated because of the limitations of the power source voltage.
Thus, the tradeoffs between the gain, the frequency characteristic and input/output dynamic ranges limits the amplifier designer's freedom. Further, because, the load resistance of prior amplifier circuits was not variable maximizing the S/N ratio of such amplifiers resulted in a decrease in gain.
For the above reasons, amplifier designers often connect multiple amplifier circuits in cascade in order to obtain a suitable gain, however as a result of using multiple stages the S/N and DG DP of such a circuit decrease relative to a single stage circuit, while the power consumption increases relative to a single stage circuit.